2 weeks ago I posted a question on a rear diff swap and got some good answers, I now have a couple more.
I own 1984 300d turbo cuurently with a 3.07 ratio, I have located a 2.88 ratio diff that I will pick up. I also talked to a local trans guy who does alot of german trans and said that the driven gear would be no problem to change so that the speedo will read correctly.
Today after crunching the numbers, my math sucks, but I think I came up with the correct numbers as close as I can get them. A swap from a 3.07 to a 2.88 rear will give me a 6.25% decrease in rpms. I know already that at 65mph I am turning 3000rpms. so now I will be at 2815rpms approx. and at 75mph I was normally at about 3465rpms I will now go down to 3245rpms approx. It is not much of a gain, though I thought I could do better. my goal is for less noise at highway speeds and at least a 10% gain in fuel economy. I go cross country every so often an looking to get this thing right. next trip is to Denver in late Sept.
Question 1, is it possible to go even lower in ratio as my friends 92' 2.5 turbo has a 2.65 ratio?
are my calculations correct? would it be worth it. does 6.25%rpm decrease translate into a 6% savings in fuel?
any advice is greatly appreciated as this is the premier forum for great answers.
thanks
Peter